Methods of forming field oxide and active area regions on a semiconductor substrate

ABSTRACT

Methods of forming a field oxide region and an adjacent active area region are described. A semiconductive substrate is masked with an oxidation mask while an adjacent area of the substrate remains unmasked. The substrate is exposed to conditions effective to form a field oxide region in the adjacent area. The field oxide region has a bird&#39;s beak region which extends toward the active area. A mass of material is formed over at least a portion of the bird&#39;s beak region. In a preferred implementation, the mass of material is formed from material which is different than the material from which the oxidation mask and the field oxide region are formed. According to one aspect of the invention, the material comprises polysilicon. In another preferred implementation, such different material comprises a spacer which is formed over at least a portion of the oxidation mask. Preferably, an undercut region is formed under the mass or spacer and subsequently filled with oxide material. During the filling of the undercut region, at least some of the mass or spacer is oxidized to form a bump over the bird&#39;s beak region. Oxide material is then removed from over the active area with such removal reducing the size of the bump. A gate dielectric layer can then be provided over the active area.

TECHNICAL FIELD

[0001] This invention relates to methods of forming field oxide and adjacent active area regions on a semiconductive substrate.

BACKGROUND OF THE INVENTION

[0002] Integrated circuits are typically fabricated over a semiconductive substrate and can include many individual transistor or device constructions. Implementing electrical circuits involves connecting isolated devices through specific electrical paths. It must, therefore, be possible to isolate respective transistor or device constructions. A variety of techniques have been developed to isolate devices in integrated circuits. One technique, termed LOCOS isolation (for LOCal Oxidation of Silicon) involves the formation of a semirecessed oxide in the nonactive (or field) areas of the substrate. Prior art LOCOS isolation is discussed briefly in this section as such pertains to the present invention. For a more detailed discussion of LOCOS isolation, the reader is directed to a text by Wolf entitled, “Silicon Processing for the VLSI Era”, Vol. 2, Chapter 2, the disclosure of which is hereby incorporated by reference.

[0003] Referring to FIG. 1, a prior art semiconductor wafer fragment in process is indicated generally by reference numeral 10. Such comprises a semiconductive substrate 12 over which a field oxide or isolation oxide region is to be formed by LOCOS techniques. In the context of this document, the term “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to bulk semiconductive material such as semiconductive wafer (either alone or in assemblies comprising other materials thereon) and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. A pad oxide layer 14 is formed over substrate 12 and an oxidation mask 16, comprised of a suitable material such as silicon nitride, is formed over pad oxide layer 14. Portions of layers 14, 16 (not shown) have been removed to expose a substrate portion 18. Portion 18 constitutes a portion of the substrate in which a LOCOS isolation structure or isolation oxide region is to be formed. Adjacent substrate portion 18, a masked substrate portion 20 remains. Portion 20 constitutes at least a portion of the substrate which is to support at least one integrated circuit construction. Accordingly, such portion constitutes an active area.

[0004] Referring to FIG. 2, substrate 12 is exposed to oxidation conditions which are sufficient to form oxide isolation region or field oxide region 22 within portion 18. Accordingly, as is known, the formation of region 22 typically causes a bird's beak region 24 to be formed, a portion of which extends under and upwardly lifts oxidation mask 16.

[0005] Referring to FIG. 3, oxidation mask 16 and pad oxide layer 14 are suitably removed or etched. Such defines a more pronounced bird's beak in region 24.

[0006] Referring to FIG. 4, a sacrificial oxide layer 26 is formed over substrate 12 typically to overcome a phenomena known as the Kooi effect. During the growth of field oxide, the Kooi effect can cause later defects when a gate oxide is formed. More specifically, during field oxide growth, a thin layer of silicon nitride can form on the silicon surface and close to the border of the active regions as a result of the reaction between the oxidizing species, oxygen and water, and the silicon nitride. In particular, NH₃ is generated from the reaction between the water and the masking nitride during the field oxidation step. This NH₃ then diffuses through the oxide and reacts with the silicon substrate to form siliconnitride ribbons. When the nitride mask and pad oxide are removed, there is a possibility that the siliconnitride ribbon remains present. When gate oxide is subsequently grown, the growth rate becomes impeded at the locations where such siliconnitride ribbons remain. The gate oxide is thus thinner at these locations than elsewhere giving rise to problems associated to low voltage breakdown of the gate oxide. The most widely used method of eliminating the siliconnitride ribbon problem is to grow a sacrificial oxide layer, typically about 25 to about 50 nanometers thick, after stripping the masking nitride and pad oxide. This sacrificial oxide layer is then removed by wet etching before growing the final gate oxide.

[0007] It has been found that the removal of the prior formed pad oxide layer 14 (FIG. 2) together with the formation and removal of the sacrificial oxide layer 26 can lead to a slightly thinner region 28 adjacent oxide isolation region 22 when the gate oxide layer is ultimately formed.

[0008] Referring to FIG. 5, and prior to formation of a gate oxide layer, 4 sacrificial oxide layer 26 is suitably removed. In the illustrated example, such can cause field oxide region 22 to be recessed to a degree which results in the formation of a convex bump 30 laterally adjacent oxide isolation region 22.

[0009] Referring to FIG. 6, a gate oxide layer 32 is formed over substrate 12. Oxidation of bump 30 results in localized thinning of the gate oxide within region 28. Such thinning can lead to device failure brought on by gate shorting.

[0010] This invention arose out of concerns associated with improving the processing of semiconductor devices. This invention also arose out of concerns associated with improving the uniformity with which semiconductor devices can be formed.

SUMMARY OF THE INVENTION

[0011] Methods of forming a field oxide region and an adjacent active area region are described. A semiconductive substrate is masked with an oxidation mask while an adjacent area of the substrate remains unmasked. The substrate is exposed to conditions effective to form a field oxide region in the adjacent area. The field oxide region has a bird's beak region which extends toward the active area. A mass of material is formed over at least a portion of the bird's beak region. In a preferred implementation, the mass of material is formed from material which is different than the material from which the oxidation mask and the field oxide region are formed. According to one aspect of the invention, the material comprises polysilicon. In another preferred implementation, such different material comprises a spacer which is formed over at least a portion of the oxidation mask. Preferably, an undercut region is formed under the mass or spacer and subsequently filled with oxide material. During the filling of the undercut region, at least some of the mass or spacer is oxidized to form a bump over the bird's beak region. Oxide material is then removed from over the active area with such removal reducing the size of the bump. A gate dielectric layer can then be provided over the active area.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Preferred embodiments of the invention are described below with reference to the following accompanying drawings.

[0013]FIG. 1 is a diagrammatic sectional view of a semiconductor wafer fragment at one prior art processing step and is discussed in the “Background of the Invention” section above.

[0014]FIG. 2 is a view of the FIG. 1 wafer fragment at a prior art processing step subsequent to that shown by FIG. 1.

[0015]FIG. 3 is a view of the FIG. 1 wafer fragment at a prior art processing step subsequent to that shown by FIG. 2.

[0016]FIG. 4 is a view of the FIG. 1 wafer fragment at a prior art processing step subsequent to that shown by FIG. 3.

[0017]FIG. 5 is a view of the FIG. 1 wafer fragment at a prior art processing step subsequent to that shown by FIG. 4.

[0018]FIG. 6 is a view of the FIG. 1 wafer fragment at a prior art processing step subsequent to that shown by FIG. 5.

[0019]FIG. 7 is a view of a semiconductor wafer fragment at a processing step which corresponds to the FIG. 2 processing step.

[0020]FIG. 8 is a view of the FIG. 7 wafer fragment at a subsequent processing step in accordance with the invention.

[0021]FIG. 9 is a view of the FIG. 8 wafer fragment at a processing step subsequent to that shown by FIG. 8.

[0022]FIG. 10 is a view of the FIG. 8 wafer fragment at a processing step subsequent to that shown by FIG. 9.

[0023]FIG. 11 is a view of the FIG. 8 wafer fragment at a processing step subsequent to that shown by FIG. 10.

[0024]FIG. 12 is a view of the FIG. 8 wafer fragment at a processing step subsequent to that shown by FIG. 11.

[0025]FIG. 13 is a view of the FIG. 8 wafer fragment at a processing step subsequent to that shown by FIG. 12.

[0026]FIG. 14 is a view of the FIG. 8 wafer fragment at a processing step subsequent to that shown by FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).

[0028] Like numbers from the abovedescribed embodiment are utilized where appropriate, with differences being indicated with the suffix “a” or with different numerals. Processing of the below-described semiconductor wafer fragment takes place in accordance with the above described methodology up to and including that which is shown in FIG. 2. It is at this point that the novel departure begins.

[0029] Referring to FIG. 7, a semiconductor wafer fragment 10 a is shown at a processing step following that which is shown in FIG. 1. Accordingly, a pad oxide layer 14 a and an oxidation mask or masking layer 16 a, preferably comprising nitride, have been suitably formed and subsequently etched, as in FIG. 1, to define thereunder a substrate active area portion 20 a. A laterally adjacent area 18 a is unmasked with any masking layer material which enables a field oxide or isolation oxide region 22 a to be formed as described above. As shown, masking layer 16 a includes a sidewall 17 adjacent unmasked substrate area 18 a.

[0030] Substrate 12 a is subjected or exposed to oxidizing conditions which are effective to form the oxide isolation region 22 a in area 18 a adjacent masking layer 16 a and masking layer sidewall 17. Oxide isolation region 22 a includes a bird's beak region 24 a which extends toward masking layer sidewall 17 and joins with active area portion 20 a.

[0031] Referring to FIG. 8, a layer 34 is formed over masking layer 16 a and oxide isolation region 22 a. Preferably, layer 34 comprises a material which is different from either of the materials from which oxidation mask 16 a or field oxide region 22 a are formed. In the illustrated example, layer 34 constitutes a nonoxide material as deposited which can be subsequently oxidized. A suitable and preferred material is polysilicon.

[0032] Referring to FIG. 9, layer 34 material is selectively removed, as by suitable chemical etching, relative to masking layer 16 a and oxide isolation region 22 a to leave a mass 36 of oxidizable material over at least a portion of bird's beak region 24 a and laterally adjacent oxidation mask sidewall 17. An exemplary dry etch chemistry includes chlorine (Cl₂) by itself, or a mixture which includes two or more of the following to improve selectivity to oxide: chlorine (Cl₂), hydrogen is bromide (HBr), helium (He), and/or oxygen (O₂) In the illustrated and preferred embodiment, mass 36 comprises a spacer which is formed over at least a portion of bird's beak region 24 a to cover less than all of sidewall 17.

[0033] Referring to FIG. 10, oxidation mask 16 a is removed. In the illustrated example, layer 16 a (FIG. 9) is removed by conducting a suitable chemical etch which is substantially selective relative to both the preferred polysilicon material from which spacer 36 is formed and the oxide material from which oxide isolation region 22 a is formed. Accordingly, the removing of the masking layer leaves spacer 36 over bird's beak region 24 a. An exemplary etch chemistry includes phosphoric acid (H₃PO₄).

[0034] Referring to FIG. 11, oxide material is selectively removed relative to spacer 36. Such effectively removes pad oxide layer 14 a (FIG. 10). In the illustrated example, a chemical etch of the oxide material is conducted substantially selective relative to the preferred polysilicon material of spacer 36 and to a degree which is sufficient to remove bird's beak region material from under the spacer. An exemplary etch chemistry includes dilute hydrofluoric acid (HF). An exemplary H₂O:HF 10:1 or 100:1 ratio would be suitable. Accordingly, ii such forms an undercut or undercut region under the spacer which is illustrated as the leftmost undercut region at 38. Such etching also removes oxide material from isolation oxide region 22 a to a degree which is sufficient to remove isolation oxide region material from under the spacer. Such forms an undercut or undercut region under the spacer which is the rightmost undercut region designated at 38.

[0035] Referring to FIG. 12, some, and preferably all of undercut region 38 (FIG. 11) is filled with oxide material. In a preferred implementation, the substrate is exposed to oxidizing conditions which are sufficient to form a sacrificial oxide layer 26 a over substrate 12 a and to oxidize some, and preferably all of spacer 36 (FIG. 11) to form a bump 40 of oxidized material over bird's beak region 24 a. Accordingly, spacer 36 is preferably completely oxidized to form bump 40.

[0036] Referring to FIG. 13, sacrificial oxide layer 26 a is removed to outwardly expose an upper surface of substrate 12 a. Preferably, the sacrificial oxide layer is completely removed. In the illustrated example, at least some of bump 40 is removed as well.

[0037] Referring to FIG. 14, a gate oxide or dielectric layer 42 is formed over substrate active area 20 a.

[0038] Comparison of the prior art gate oxide or dielectric layer 32 (FIG. 6) with the improved FIG. 14 gate oxide layer 42 indicates that the latter construction provides a greater degree of uniformity of thickness. In addition, the above described methodology reduces the loss of field oxide at the interface of the active area and field oxide during removal of the pad oxide and sacrificial layers. Correspondingly, concerns associated with device performance relative to a variable or thin gate oxide region, such as region 28 of FIG. 6 are greatly reduced, if not eliminated due to reduction or elimination of bump formation in the active area.

[0039] In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents. 

1. A method of forming an oxide isolation region and an adjacent active area region on a semiconductive substrate comprising: forming an oxide isolation region over a semiconductive substrate laterally adjacent a semiconductive substrate active area, the oxide isolation region having a bird's beak region which extends to and joins with the active area; forming a mass of oxidizable material over at least a portion of the bird's beak region; and oxidizing at least some of the mass of oxidizable material to form a bump of oxidized material over the bird's beak region.
 2. The method of claim 1 , wherein the forming of the mass of oxidizable material comprises forming a polysilicon layer over the substrate and removing polysilicon layer material substantially selective relative to the oxide isolation region.
 3. The method of claim 1 , wherein the forming of the mass of oxidizable material comprises forming a polysilicon layer over the substrate and selectively etching polysilicon layer material substantially selective relative to the oxide isolation region.
 4. The method of claim 1 , wherein the oxidizing comprises completely oxidizing the mass of oxidizable material.
 5. The method of claim 1 further comprising: prior to the forming of the oxide isolation region, forming an oxidation mask over the substrate active area, the oxidation mask having a sidewall; and wherein the forming of the mass of oxidizable material comprises: forming a layer of mass material over the substrate; and selectively removing mass material relative to oxidation mask material and to a degree sufficient to leave a sidewall spacer over at least some of the oxidation mask sidewall.
 6. A method of forming field oxide and an adjacent active area region on a semiconductive substrate; masking an active area portion of the semiconductive substrate with an oxidation mask while leaving an adjacent area of the semiconductive substrate unmasked with the oxidation mask, the oxidation mask having a sidewall adjacent the unmasked substrate area; subjecting the masked substrate to oxidizing conditions effective to form a field oxide region in the adjacent area, the field oxide region having a bird's beak region extending toward the oxidation mask sidewall; forming a mass of oxidizable material over at least a portion of the bird's beak region laterally adjacent the oxidation mask sidewall; oxidizing at least some of the mass of oxidizable material to form a bump of oxidized material over the bird's beak region; and removing at least some of the bump of oxidized material.
 7. The method of claim 6 , wherein the oxidizing comprises completely oxidizing the mass of oxidizable material.
 8. The method of claim 6 further comprising: prior to the masking of the active area portion, forming a pad layer over the active area portion, the oxidation mask overlying the pad layer; and prior to the oxidizing of the mass of oxidizable material, removing the pad layer substantially selective relative to the oxidizable material.
 9. The method of claim 6 further comprising: prior to the masking of the active area portion, forming a pad layer over the active area portion, the oxidation mask overlying the pad layer; and prior to the oxidizing of the mass of oxidizable material, removing the pad layer substantially selective relative to the oxidizable material and removing bird's beak region material under the oxidizable material to a degree sufficient to undercut the oxidized material.
 10. The method of claim 6 , wherein the forming of the mass of oxidizable material comprises forming a material which is different from either of the materials from which the oxidation mask or the field oxide region are formed.
 11. The method of claim 6 , wherein: the oxidizing of the mass of oxidizable material comprises forming a sacrificial oxide layer over the active area portion; and the removing comprises etching the sacrificial oxide layer.
 12. The method of claim 6 , wherein: prior to the masking of the active area portion, forming a pad layer over the active area portion, the oxidation mask overlying the pad layer; the forming of the mass of oxidizable material comprises forming a material which is different from either of the materials from which the oxidation mask or the field oxide region are formed; prior to the oxidizing of the mass of oxidizable material, removing the pad layer substantially selective relative to the oxidizable material and removing bird's beak region material under the oxidizable material to a degree sufficient to undercut the oxidizable material; the oxidizing of the mass of oxidizable material comprises forming a sacrificial oxide layer over the active area portion; and the removing comprises etching the sacrificial oxide layer.
 13. The method of claim 6 , wherein: the masking of the active area portion comprises forming a mask comprising nitride thereover, and the forming of the mass of oxidizable material comprises forming polysilicon material over the bird's beak region.
 14. A method of forming field oxide and an adjacent active area region on a semiconductive substrate; masking an active area portion of the semconductive substrate with an oxidation mask while leaving an adjacent area of the semiconductive substrate unmasked with the oxidation mask, the oxidation mask having a sidewall adjacent the unmasked substrate area; subjecting the masked substrate to oxidizing conditions effective to form a field oxide region in the adjacent area, the field oxide region having a bird's beak region extending toward the oxidation mask sidewall; forming a spacer comprising a material which is different than the material from which the oxidation mask is formed, the spacer being formed over at least a portion of the bird's beak region laterally adjacent the oxidation mask sidewall; removing the oxidation mask and leaving the spacer over the bird's beak region; etching the field oxide region substantially selective relative to the spacer material and to a degree sufficient to form an undercut region under the spacer; and filling the undercut region with oxide material.
 15. The method of claim 14 , wherein the filling of the undercut region comprises forming a sacrificial oxide layer over the active area portion.
 16. The method of claim 14 , wherein the filling of the undercut region comprises exposing the substrate to oxidizing conditions to a degree sufficient to oxidize at least some of the spacer and to fill the undercut region.
 17. The method of claim 14 , wherein the filling of the undercut region comprises exposing the substrate to oxidizing conditions to a degree sufficient to completely oxidize the spacer and to fill the undercut region.
 18. The method of claim 14 , wherein the forming of the spacer comprises forming polysilicon over at least a portion of the oxidation mask sidewall.
 19. The method of claim 14 , wherein: the filling of the undercut region comprises forming a sacrificial oxide layer over the active area portion while oxidizing at least some of the spacer; and etching the sacrificial oxide layer to expose the active area portion, at least some of the oxidized spacer being removed by said etching.
 20. The method of claim 14 , wherein: the filling of the undercut region comprises forming a sacrificial oxide layer over the active area portion while completely oxidizing the spacer; and etching oxide material to expose the active area portion and to remove at least some of the oxidized spacer.
 21. A method of defining an isolation region and an adjacent active area on a semiconductive substrate comprising: forming an oxide isolation region over a semiconductive substrate laterally adjacent a semiconductive substrate active area; forming a spacer comprising a nonoxide material over at least a portion of the isolation region; etching oxide material of the isolation region selectively relative to the nonoxide material and to a degree sufficient to form an undercut region under the spacer; forming a sacrificial oxide layer over the substrate and oxidizing at least a portion of the spacer and filling the undercut region with oxide material; removing at least some of the sacrificial oxide layer to outwardly expose the substrate active area; and forming a gate oxide layer over the substrate active area.
 22. The method of claim 21 , wherein the removing of the sacrificial oxide layer comprises completely removing the sacrificial oxide layer.
 23. The method of claim 21 , wherein the forming of the spacer comprises forming the spacer from polysilicon material.
 24. The method of claim 21 , wherein the forming of the sacrificial oxide layer comprises completely oxidizing the spacer.
 25. The method of claim 21 further comprising prior to the forming of the isolation region, forming an oxidation mask comprising nitride material over the active area of the substrate, the oxidation mask having a sidewall; and wherein the forming of the spacer comprises forming polysilicon material over at least a portion of the sidewall.
 26. The method of claim 21 further comprising prior to the forming of the isolation region, forming an oxidation mask comprising nitride material over the active area of the substrate, the oxidation mask having a sidewall; wherein the forming of the spacer comprises forming polysilicon material over at least a portion of the sidewall; and wherein the forming of the sacrificial oxide layer comprises completely oxidizing the spacer.
 27. A method of forming an isolation region comprising: forming a layer comprising oxide material over a semiconductive substrate surface; forming a nitride masking layer over the oxide material layer; removing at least some of the nitride masking layer, the removing leaving nitride masking layer material over an active area portion of the semiconductive substrate while leaving an adjacent area of the semiconductive substrate unmasked with masking layer material, the nitride masking layer having a sidewall adjacent the unmasked substrate area; exposing the substrate to oxidation conditions effective to form an oxide isolation region in the adjacent area, the oxide isolation region having a bird's beak region extending toward the nitride masking layer sidewall; forming a oxidizable spacer over a portion of the isolation region adjacent the nitride masking layer sidewall; removing the nitride masking layer and leaving the spacer over the isolation region; etching oxide material substantially selective relative to the material from which the spacer is formed, the etching removing the layer comprising oxide material and oxide material underlying the spacer; forming a sacrificial oxide layer over the substrate, the forming oxidizing at least some of the spacer; removing at least some of the sacrificial oxide layer over the substrate active area; and forming a gate oxide layer over the substrate active area.
 28. The method of claim 27 , wherein: the forming of the sacrificial oxide layer completely oxidizes the spacer; and the removing of the sacrificial oxide layer over the substrate active area comprises completely removing the sacrificial oxide layer from over the substrate active area.
 29. The method of claim 27 , wherein the forming of the oxidizable spacer comprises forming a layer comprising polysilicon over the substrate and etching the layer to a degree sufficient to leave the spacer over the isolation region.
 30. A method of forming an isolation region comprising: forming a pad oxide layer over a semiconductive substrate surface; forming a nitride masking layer over the pad layer; etching at least some of the nitride masking layer to provide a masking layer sidewall adjacent a substrate area in which an oxide isolation region is to be formed, at least some of the nitride masking layer which is not etched overlying an adjacent substrate active area; exposing the substrate to oxidation conditions effective to form an oxide isolation region in the substrate area, adjacent the masking layer sidewall, the oxide isolation region having a bird's beak region extending ii toward the masking layer sidewall; forming a layer comprising polysilicon over the nitride masking layer and the oxide isolation region; selectively removing polysilicon material relative to the nitride is masking layer and the oxide isolation region to leave a polysilicon spacer over at least a portion of the bird's beak region and covering less than all of the nitride masking layer sidewall removing the nitride masking layer and leaving the polysilicon spacer over the at least a portion of the bird's beak; etching oxide material substantially selective relative to the polysilicon spacer, the etching removing the pad oxide layer and at least some oxide material underlying the spacer; forming a sacrificial oxide layer over at least a portion of the substrate, the forming oxidizing at least some of the polysilicon spacer to form a bump of oxidized material over the at least a portion of the bird's beak region; removing a) at least some of the sacrificial oxide layer over the substrate active area and b) at least some of the bump of oxidized material; and forming a gate oxide layer over the substrate active area. 